Field of the Invention
The present invention relates to a wire electric discharge machine, and more particularly, to a wire electric discharge machine for taper-machining a tilted workpiece.
Description of the Related Art
In a conventional machining method using a wire electric discharge machine, as shown in FIG. 1, a voltage is applied to a wire electrode 4 stretched between an upper wire guide 12 in an upper nozzle 10 and a lower wire guide 13 in a lower nozzle 11, and a workpiece 3 fixed on a workpiece table 1, which is driven relative to the wire electrode 4, is moved along two axes, X- and Y-axes (not shown), which perpendicularly intersect each other on a horizontal plane. Grooving is performed to obtain a desired machining shape by melting and removing the workpiece 3 by continuous electric discharge caused (in a machining gap) between the wire electrode 4 and the workpiece 3. The workpiece 3 thus melted and removed is reduced to sludge, which is discharged from a groove by a working fluid ejected from the upper and lower nozzles 10 and 11.
An example of conventional taper machining will be described with reference to FIGS. 2 to 5. In this example, as shown in FIG. 3, a member having a 45-degree slope is cut out of a cuboid workpiece.
In taper-machining the cuboid workpiece shown in FIG. 3, the horizontal position (XY-position) of the upper wire guide 12 relative to the lower wire guide 13 is shifted so that the wire electrode 4 is stretched obliquely relative to a workpiece mounting surface (not shown). This taper machining is a well-known method used to obtain a conical or quadrangular pyramidal shape or some other shape with arbitrary slopes.
The amount of movement of the upper wire guide 12 relative to the lower wire guide 13 can be determined by calculation. In general, the upper and lower wire guides are formed of members called “die guides” or “wire die guides” having guide holes through which the wire electrode 4 is passed.
In a machining program (O0001) shown in FIG. 4, a taper machining function is enabled by a command code M15, and a coordinate system for machining programs and a machining start point (0,−5) are set by G92. At the machining start point, the wire electrode 4 is in a posture perpendicular to a table surface (XY-plane). In response to a command G01X-5.0, the wire electrode 4 starts to move toward a point A. The moment this movement is started, the posture of the wire electrode 4 starts to tilt to prepare for slope machining in the next block (linear block A-B shown in FIG. 3), in response to commanded G51 (command for a leftward tilt of the wire) and T45.0 (in which T is a command for a tilt angle of the wire electrode 4). The tilt angle of the wire electrode 4 becomes 45° when the point A is reached.
When Y60.0 is then commanded, the wire electrode 4 is kept at 45° on the left side with respect to the direction of movement as it starts and continues slope machining and advances to a point B. Finally, in response to a command X5.0, the wire electrode 4 starts to move toward a machining end point. The moment this movement is started, the tilt of the wire electrode 4 starts to be gradually restored to its original angle in response to commanded G50 (command for canceling the tilt of the wire electrode 4) and T0. When the wire electrode 4 reaches the machining end point, it is restored to its vertical state (with the tilt angle at 0°), whereupon the machining ends.
While the workpiece has a single taper angle in the example described above, a workpiece having a plurality of taper angles is machined in the following manner.
In machining the workpiece with two different tapers of 45° and 25°, as shown in FIG. 5, the wire electrode 4 is tilted at 45° at the maximum from the vertical state according to the conventional taper machining. FIG. 6 shows a program example (O0002) for machining of the workpiece with the tapers of 45° and 25°.
The taper machining function is enabled by the command code M15, and the coordinate system for the machining programs and the machining start point (0,−5) are set by G92. At the machining start point, the wire electrode 4 is in a posture perpendicular to the table surface (XY-plane). In response to the command G01X-5.0, the wire electrode 4 starts to move toward the point A. The moment this movement is started, the posture of the wire electrode 4 starts to tilt to prepare for slope machining in the next block (linear block A-C shown in FIG. 5), in response to the commanded G51 and T45.0. The tilt angle of the wire electrode 4 becomes 45° when the point A is reached by the wire electrode 4.
When Y30.0 is then commanded, the wire electrode 4 is kept at 45° on the left side with respect to the direction of movement as it starts and continues slope machining and advances to a point C. When the wire electrode 4 reaches the point C, its tilt angle becomes 25° in response to a command T25.0. When Y30.0 is then commanded, the wire electrode 4 is kept at 25° with respect to the direction of movement as it starts and continues slope machining and advances to the point B. Finally, in response to the command X5.0, the wire electrode 4 starts to move toward the machining end point. The moment this movement is started, the tilt of the wire electrode 4 starts to be gradually restored to its original angle in response to the commanded G50 and T0. When the wire electrode 4 reaches the machining end point, its posture is restored to the vertical state (with the tilt angle at 0°), whereupon the machining ends.
As described above, the necessary tilt angle of the wire electrode 4 for the execution of the taper machining corresponds to the tilt angle (taper angle) of a machined surface and is usually designated by a numerical value in a machining program. With the workpiece mounting surface assumed to be on the XY-plane (Z=0), for example, the tilt angle is designated as follows:                “45° to Z-direction=T45.0”, or        “25° to Z-direction=T25.0”.In order to adjust the actual tilt of the wire electrode 4 to the designated tilt angle, the position (relative XY-position) of the upper wire guide 12 relative to the lower wire guide 13 is shifted from a position where the wire guides 12 and 13 are vertically aligned to a position where the wire electrode 4 correctly extends along a plane with the programmed tilt. This movement of the wire electrode 4 is achieved by moving (along a U-axis parallel to the X-axis and a V-axis parallel to the Y-axis) a drive unit that supports one (e.g., upper wire guide 12) of the wire guides 12 and 13.        
(1) Japanese Patent Application Laid-Open No. 2-139129 discloses a method of parallelism correction for a wire electric discharge machine, whereby the parallelism of a workpiece on an XY-table relative to the X- and Y-axes is corrected. In this correction method, the workpiece is mounted on the table, its parallelism is measured, and the wire guide is moved based on the resulting measured value so that the wire electrode extends perpendicular to the workpiece. According to this correction method, the mounting of the workpiece need not be adjusted, so that labor and time for adjustment can be saved and machining setup can be considerably simplified. For machining, moreover, a controller is provided with correction means, and the distance covered by the relative movement of the workpiece and the wire electrode is adjusted to a command value. Thus, accurate machining can be achieved.
Specifically, the above-described patent document discloses a technique for correcting the parallelism so that the wire electrode extends perpendicular to the workpiece mounted on at an arbitrary angle on the surface of the table, thereby transforming a coordinate system for machining programs, and performing wire electric discharge machining according to the corrected machining command value, based on the transformed coordinate system. This technique is characterized in that even if the workpiece is mounted at any angle on the table surface, machining can be achieved in the same manner as in the case where the workpiece is mounted parallel on the table surface. However, this technique is designed to enable machining without a complicated setup operation after the workpiece is mounted on the table and is based on the assumption that the workpiece is mounted on the table surface. In this document, there is neither description nor suggestion of various adverse effects in taper machining, as well as of how the workpiece is mounted at a deliberate angle on the table surface.
(2) Japanese Patent Application Laid-Open No. 2000-52153 discloses a technique to devise the shape of a wire guide, thereby solving the problems of disconnection of a wire electrode and streaks formed on a machined surface during taper machining. However, the technique disclosed in this patent document relates to the shape of a wire guide that is based on the assumption that the wire electrode is stretched at an angle. Therefore, the above countermeasure is unnecessary if the wire electrode is stretched perpendicular to the table surface.
(3) Japanese Patent Application Laid-Open No. 2006-55923 discloses a technique to devise the shape of a wire guide so that a good machined surface can be obtained and lest a wire electrode vibrate even during wide-angle taper machining. However, the technique disclosed in this patent document also relates to the shape of a wire guide that is based on the assumption that the wire electrode is stretched at an angle. Therefore, the above countermeasure is unnecessary if the wire electrode is stretched perpendicular to the table surface.
(4) Japanese Patent Application Laid-Open No. 9-267219 discloses a wire electric discharge machine and control means configured to control the machine by numerical control. The wire electric discharge machine has a taper machining function and comprises a rotary indexing axis capable of indexing and positioning. This machine uses a machining device capable of automatically machining a forming tool having a desired cutting edge shape with high efficiency and high accuracy. The machining device machines a cutting edge forming portion of the material on which a basic shape has already been machined so as to obtain a desired cutting edge shape and a flank face. The material of the forming tool is mounted as a workpiece on the rotary indexing axis of the wire electric discharge machine. Electric discharge machining of a cutting edge and a flank face on the cutting edge forming portion of the forming tool is automatically performed by controlling the relative movement of a wire electrode and the workpiece, based on a numerical control program created according to the desired cutting edge shape to be obtained in advance.
In the technique disclosed in the patent document described above, the workpiece is mounted on the rotary axis parallel to a table surface, and the rotary axis is rotated as the cutting edge shape and the flank face are machined. Since the workpiece itself moves around the rotary axis, however, this technique is not designed to correct a coordinate system or a command value for machining in accordance with the tilt of the workpiece. Taper machining is based on a conventional system such that an orthogonal coordinate system based on the table surface is used, and the wire electrode for the taper machining is tilted relative to its vertical running posture.
(5) Japanese Patent Application Laid-Open No. 2006-159396 discloses a technique in which three-dimensional positions of three points that are not on a single straight line on the upper surface of a workpiece is measured, a direction perpendicular to the tilted workpiece is calculated based on the three measured points, and a wire electrode is positioned so that it extends perpendicular to the workpiece. Based on this, according to this technique, moreover, the position of the wire electrode is controlled to determine a tilt commanded by a machining program, whereby an error produced when the workpiece is set in position is corrected.
In order to correct an error in the posture of the workpiece to be mounted, according to the technique disclosed in the patent document described above, however, the vertical position of the wire electrode is adjusted so that the wire electrode extends perpendicular to the workpiece. This technique is not designed to deliberately tilt the workpiece to be taper-machined so that the wire electrode extends substantially perpendicular to a table surface.
In the case of the wire electric discharge machine, the taper machining in which the wire electrode 4 supported by the wire guide portion is stretched obliquely by translating the upper wire guide 12 relative to the table surface involves more technical problems than vertical machining in which the wire electrode 4 is stretched vertically. For example, the problems are as follows:
(I) The machining speed cannot be increased due to difficulty in discharging sludge.
(II) The wire guide applies frictional force to the wire electrode, thereby adversely affecting the machining accuracy (surface roughness).
(III) Since supporting points of the wire electrode bent by the wire guide vary depending on the shape precision and machining state of the wire guide, high-precision machining is difficult.
(IV) It is difficult to set machining conditions.
(V) In some cases, the wire electrode 4 may inevitably exceed movable ranges of the upper and lower wire guides 12 and 13, so that the workpiece cannot be machined.
The following is an explanation of the above problems (I) to (V).
(I) The machining speed cannot be increased due to difficulty in discharging sludge:
In the wire electric discharge machining, electric discharge between the wire electrode 4 and the workpiece 3 is repeated as the workpiece 3 is melted and removed around the wire electrode 4. Thus, a machining groove 8 is formed as machining advances. FIGS. 7A and 7B show relationships between the wire electrode 4, workpiece 3, and working fluid 7 during the taper machining.
During the wire electric discharge machining, the working fluid 7 is ejected from the upper and lower nozzles 10 and 11 to discharge sludge from the machining groove 8 between the wire electrode 4 and the workpiece 3, thereby preventing short-circuiting between the wire electrode 4 and the workpiece 3 or disconnection of the wire electrode 4. Thus, the workpiece 3 can be efficiently machined.
FIGS. 7A and 7B are diagrams illustrating relationships between the wire electrode 4, workpiece 3, and working fluid 7.
In the vertical machining, the wire electrode 4 is stretched perpendicular to the table surface (not shown), as shown in FIG. 7A. In this case, the ejecting direction of the working fluid 7 is coincident with the running direction of the wire electrode 4, so that the working fluid 7 can smoothly flow into the machining groove 8, thereby efficiently discharging the sludge from the groove 8.
In the taper machining, in contrast, the wire electrode 4 is stretched obliquely relative to the table surface, as shown in FIG. 7B. In this taper machining, the ejecting direction of the working fluid 7 is not coincident with the running direction of the wire electrode 4, so that the working fluid 7 cannot easily flow into the machining groove 8, and therefore, the sludge cannot be efficiently discharged from the machining groove 8. The greater the tilt angle θ, the more distinct this tendency is. The table surface is perpendicular to the drawing plane of FIG. 7 and parallel to the bottom surface of the workpiece 3.
In the taper machining, moreover, the wire electrode 4 is bent by the wire guide portion as it runs, so that it may sometimes interfere with the nozzles 10 and 11 that guide the working fluid 7 for straight ejection. Accordingly, it is necessary to use nozzles with large inside diameters such that the flow velocity of the working fluid of the same flow volume is reduced.
In the case of the taper machining, therefore, the sludge discharge efficiency is degraded, so that short-circuiting or disconnection of the wire electrode 4 easily occurs. Accordingly, electric discharge machining conditions must be eased, so that machining cannot be performed at high speed.
(II) The wire guide applies frictional force to the wire electrode, thereby adversely affecting the machining accuracy (surface roughness):
In the taper machining in which the wire electrode 4 is stretched obliquely relative to the table surface as it is machined, the wire electrode 4 is suddenly bent at the wire guide portion, so that it cannot smoothly move in wire guide holes. Thus, the wire electrode 4 may be caused to vibrate and disconnected. Consequently, a machined surface may be streaked, so that the surface roughness is degraded. The greater the taper angle, the more distinct this tendency is.
In order to solve these problems, according to the techniques disclosed in Japanese Patent Application Laid-Open Nos. 2000-52153 and 2006-55923, an attempt is made to devise the shape of the wire guide portion to suppress vibration of the wire electrode 4. However, frictional force produced between the wire electrode 4 and the wire guide portion still cannot be completely eliminated.
As shown in FIGS. 8A and 8B, therefore, it is impossible to completely suppress the vibration to be produced in the wire electrode 4. FIGS. 8A and 8B are diagrams illustrating how the wire electrode is bent by the wire guide portion in the taper machining.
FIG. 8A is a diagram illustrating how the wire electrode 4 is bent at the wire guide portion when a wire guide 13a with a small curvature radius is used. If the wire guide of this type is used, the wire electrode 4 cannot be easily bent, so that its vibration cannot be suppressed, and hence, the electric discharge machining is unstable. On the other hand, FIG. 8B is a diagram illustrating how the wire electrode 4 is bent at the wire guide portion when a wire guide 13b with a large curvature radius is used. If the wire guide of this type is used, the wire electrode 4 easily bends, so that the machining is stable. In this case, however, supporting points are subject to a large error, which is a substantial problem, as described later.
(III) Since supporting points of the wire electrode bent by the wire guide vary depending on the shape precision and machining state of the wire guide, high-precision machining is difficult:
An angle command method for the taper machining will be described with reference to FIG. 9. The amount of movement of the upper wire guide 12 can be calculated based on the tilt angle θ of the wire electrode 4 and a distance H between the upper and lower wire guides 12 and 13. In the case of the technique disclosed in Japanese Patent Application Laid-Open No. 2006-55923 described before, the vibration of the wire electrode 4 can be reduced by using a wire guide with a large curvature radius. In this case, however, a supporting point error occurs causing a great problem regarding precision. FIG. 10 is a diagram illustrating a supporting point error produced in the wire guide with a large curvature radius. Since the supporting point positions of the wire electrode 4 vary depending on the taper angle, as shown in FIG. 10, the tilt angle (taper angle) of the wire electrode 4 is deviated unless the supporting point error is corrected.
(IV) It is difficult to set machining conditions:
In the case of the taper machining by means of the wire electric discharge machine, which has been described in connection with the problem (I), the sludge discharge efficiency is degraded, so that short-circuiting or disconnection of the wire electrode 4 easily occurs. Conventionally, the electric discharge machining conditions must be eased to overcome this. If the machining conditions are eased, however, the machining speed is reduced correspondingly, resulting in an increase in machining time, although the possibility of disconnection of the wire electrode 4 is reduced. Thus, efficient electric discharge machining is expected to be performed without easing the machining conditions, if possible.
Since the sludge discharge efficiency varies depending on the taper angle, however, it is actually very difficult to adjust the degree to which the machining conditions are eased for the angle concerned.
(V) In some cases, the wire electrode 4 may inevitably exceed movable ranges of the upper and lower wire guides 12 and 13, so that the workpiece cannot be machined:
If the wire electrode 4 is tilted relative to the table surface such that the commanded taper angle is great or if the distance between the upper and lower wire guides 12 and 13 is long, as shown in FIG. 11, the amount of movement of the guide 12 and/or 13 increases. Thus, the movable range of the guide may be exceeded so that machining is prevented.
The above-described problems (I) to (V) are caused when the wire electrode 4 is stretched obliquely. The greater the taper angle, the more prominent these problems tend to be.